This invention relates in general to microwave synthesizers and in particular, to a synthesizer capable of providing adjustable of high fundamental frequencies, such as millimeter waves. Conventional synthesizers typically include a phase-locked loop which comprises a voltage controlled oscillator, a mixer, a phase detector, and a low pass filter in the loop. An external reference frequency is applied to the phase detector so that deviations of the voltage controller oscillator (VCO) from the reference frequency cause the phase detector to generate an error voltage which when applied to the VCO, adjusts the VCO frequency to match the reference frequency.
Some frequency synthesizer systems proposed include multiple-loop phase-locked loops, such as that disclosed by Mower et al. in U.S. Pat. No. 4,839,603. The multiple-loop system of Mower et al. includes an upper loop which is similar to that of conventional single loop frequency synthesizers described above as well as a lower loop for generating the reference frequency. Mower et al. employs a frequency multiplier and a mixer coupling the outputs of the upper and lower loops in order to extend the frequency range of the outputs of the synthesizer to about 20 GHz.
As known to those skilled in the art, frequency synthesizers of medium output power (up to 30 dbm) for providing millimeter waves (defined as microwave of frequencies above about 26 GHz) are difficult to make and are expensive, especially if additional amplifiers are required. This type of medium power millimeter wave synthesizers are useful, for example, as local oscillators in short haul millimeter wave links. Solutions proposed up to now frequently require expensive components such as amplifiers capable of amplifying millimeter wave frequencies. It is therefore desirable to provide improved frequency synthesizer designs capable of providing high frequency microwaves at reasonable cost. It is also desirable to provide improved frequency synthesizer designs which are simpler and less expensive than conventional designs, even at frequencies below those of millimeter waves.